Topic: Newb seeking advice.

[yesplease]

I picked up an 84 Honda Nighthawk , an 85 parts bike, and a supposedly rebuilt set of carbs for free some time ago and was wondering if the 84 would make for a decent project bike or if these bikes were too finicky to bother fixing up. I ask because I've seen a couple reviews saying the bike had gasket issues and was prone to throwing rods and was hoping someone here could shed some light on these "problems".

[KB02]

Well, First of all, what size is the engine?

I had an '83 Nighthawk 750 that ran really well. It had some electrical issues, but mechanically it was sound.

I've always liked the Nighthawks. I feel it was one of Honda's worse desisions to just leave the bike basically alone. I feel the 919 (CB900F) and the 599 (CB600F) should both bear the NightHawk name, but that's my opinion.

[yesplease]

That info would probably helpful. 
They're both 650s, which seems to be a 655cc DOHC four. And apparently the 84 model has a 5+OD transmission. I wonder if this is a true overdrive?

Edit- 4400rpm@60mph in 6th, probably not!

[TomC]

How do you define "a true overdrive"?
     One way would be an overall overdrive were the rear tire is turning faster than the engine. Not very likely.
     Another way would be transmission overdrive were the transmission output shaft is turning faster that the transmission input shaft. My 250 Yamaha does this in both fifth & sixth gear and turns about 6,000 rpm at 60 mph in sixth gear. Most motorcycles are underdriven on both the primary and secondary drive. So with a little overdrive in the transmission the overall gearing is still very much underdriven.
          TomC

That info would probably helpful. 
They're both 650s, which seems to be a 655cc DOHC four. And apparently the 84 model has a 5+OD transmission. I wonder if this is a true overdrive?

Edit- 4400rpm@60mph in 6th, probably not!

[yesplease]

Having the wheel spinning faster than the engine was what I thought of as the definition of overdrive, less than 1:1. But like you said, from the perspective of the transmission, having fifth and sixth spinning slower than the input seems to fit that as well. What do you mean by primary and secondary drive? The underdriven transmission and underdriven read cog?

[TomC]

Hi Yesplease
     By underdriven I mean that the output shaft turns slower that the input shaft or that the gear ratio is greater that one. Typically a motorcycle has a chain or gear drive between the engine and the transmission. That is the primary drive. The secondary drive is usually the chain drive to the rear wheel.
     On the CB550K3 the primary ratio is 3.063. The fifth gear ratio is 0.900. The secondary drive or chain or final reduction is 2.176, drive or front sprocket 17T and driven or rear sprocket 37t. Giving an overall ratio of 5.999. From page 181 of Honda 500/550 shop manual. In this case the transmission ratio 0.900 is an overdrive but the primary, secondary, and overall are all underdriven.
          TomC

Having the wheel spinning faster than the engine was what I thought of as the definition of overdrive, less than 1:1. But like you said, from the perspective of the transmission, having fifth and sixth spinning slower than the input seems to fit that as well. What do you mean by primary and secondary drive? The underdriven transmission and underdriven read cog?

[yesplease]

So the primary drive would be similar to the final drive in a car, the secondary drive is the chain ratio, like on a bicycle, and the transmission ratio is the third drive ratio. So when someone says overdrive they specifically mean a transmission ratio less than one. This is totally off base, but do you suppose this 650cc engine could make enough power to fire on two out of four cylinders from ~4000rpm@55mph to ~4800rpm@65mph (or whatever the proper engine speed/vehicle speed ratios are, provided it didn't shake itself apart, with throttle body injection) on a pretty flat grade?